Indomethacin and other anti-inflammatory drugs inhibited growth of rat hepatoma cells (HTC) and nontransformed human diploid fibroblasts in the G1 phase of the cell cycle. Synchronous growth of both lines occurred after removal of drug as indicated by the sequence of changes in (3H)-thymidine incorporation into DNA, cellular DNA content, mitotic index, and cell number. All (98 percent) of the cells resumed DNA synthesis following the removal of indomethacin. Further studies revealed that metabolic processes of the cell were altered in a selective manner. The active transport (system A) but not the facilitated transport of amino acids (system L) were suppressed in the presence of indomethacin. Other transport systems investigated, those of the nucleotides and deoxyglucose, were unaffected. The effects on transport were apparent within 1 hour and disappeared rapidly once drug was removed. ATP levels, which declined during the growth of control cultures, fell little in cultures arrested with indomethacin.